joricam:
I need an url, a link to add product to cart, but with attributes, say the product id is "x" and product has 1 attribute, flavour, wiht 3 posible values (cho, vanilla and strawberry), I need a link that would add the product to the cart with vanilla selected, and then direct you to the shopping cart.
You cannot do that with a URL. Attribute-selections are not auto-selectable via any URL parameters. Only via <form> POST data.
You would need to build a <form> with all the corresponding <input> fields embedded in it with all your predefined values. You would have to reverse-engineer that by studying the dynamically-generated product page and recreating the HTML for your very unique needs. And remember that when you do that you cannot change those attributes in the admin without it affecting the <form> data, and then you'll have to manually rebuild that <form> again after making those edits.
Why are you trying to skip the main product page in Zen Cart, when it already intelligently does all the magic you're looking for? If it's because you want the page to look different, then just style it according to your needs instead of reinventing the wheel.